Spatial light modulators (SLM) are used for dynamic spectral equalization of Dense Wavelength Division Multiplexing (DWDM) signals. In such systems, it is desirable to have so-called “continuous spectrum” capability in which there are no (or minimal) gaps in the SLM response. In addition it is desired to have very fine granularity, so that the wavelength channels can be flexibly defined. Furthermore, it is also desirable to be able to reconfigure the wavelength channels under software control, such that a telecommunication carrier central office can redefine the wavelength channels as needed.
Prior art approaches for performing the above described desired capabilities that are known in the art include, but are not limited to: (1) Texas Instruments' (TI) Digital Micromirror Display (DMD), (2) Grating Light Valve (GLV), (3) Polychromix's version of the GLV, (4) LightConnect's version of an SLM; and (5) Kodak's GEM interferometric SLM.
Disadvantages of these prior art solutions will be outlined below. For instance, with respect to the first prior art approach, TI's Digital micromirror fabrication process requires a very complicated and lengthy process, thereby the produced devices are expensive and additionally are generally only produced in TI's foundry. The mirrors in the TI design must be used as a diffraction grating, with the light input and output at a fixed angle that varies with wavelength.
The second approach utilizing grating light valves (GLV) has serious Polarization Mode Dispersion (PMD) signifying that different polarization modes may see different attenuation or delay through the system due to the narrow conductive stripes that create a preferred polarization. In addition, several pairs of stripes are required in each wavelength channel, which in turn places a lower limit to the size of the channels. Furthermore, the many gaps between stripes induce both scattering and loss of light, adding to insertion loss and limiting the maximum blocking attenuation.
The third prior art approach, the Polychromix GLV device similarly has all the drawbacks of the GLV described above. Its only advantage over the GLV is that the Polychromix stripes move linearly whereas the GLV stripes deform into an arc.
The fourth prior art approach by LightConnect has the disadvantage that it is an interferometric device, and hence, there will likely be difficulty holding −40 dB attenuation over environmental disturbances.
All the interferometric devices, including GLV, Polychromix, and Light-Connect, have great difficulty achieving −40 dB attenuation (blocking) over environmental changes such as temperature, aging, and laser power. This is generally thought to be caused by the interference minimum being extremely narrow and varying with wavelength.
A new approach is needed that preferably does not include an interferometric device, provides that the attenuation versus control voltage is essentially monotonic, and provides that an increase in tilt produces more attenuation. With no narrow minima to hit, 40 dB attenuation (blocking) of individual channels can be easily maintained or exceeded.